1. Technical Field
This invention relates to apparatus for the injection molding of plastic and rubbery materials. More particularly, the invention is directed to an injection assembly adapted to be used on such apparatus to thereby convert a molding material from solid particles or pellets into a continuous semi-fluid or fluid mass and to subsequently inject the same into one or more cavities defined in two mating molds.
2. Prior Art
Various apparatus of the character mentioned are known, a typical example of which is built with an upright heating cylinder, a nozzle unit attached to a bottom end of the cylinder, an injection plunger formed to be solid and supported centrally upwardly of the cylinder for up-and-down movement and a feed hopper located to communicate with the cylinder. Fitted centrally of the nozzle unit is a torpedo provided at its exit end with a needle valve. Heaters are arranged both on a peripheral surface of the nozzle unit and within a surrounding wall of the cylinder. A pair of molds are superimposed one on the other, thereby defining a cavity therebetween, and held in cooperative releasable relation to the nozzle unit.
The foregoing apparatus is wholly unsatisfactory as it is feasible only with large heat consumption and even with long molding cycle and often with objectionable material degradation.
Generally, thermoplastic materials for instance of a pellet shape are poor in thermal conductivity and hence requisite of substantial preheat in a spacious region present in the cylinder. The preheated plastics flows through a constricted channel in the torpedo where it gets softened or plasticized with heat for injection into the mold cavity. In such instance the molding material is difficult to preheat with uniformity due primarily to that region in the cylinder being relatively large in cross-sectional area. To be more specific the molding material is subject to heat in a shorter duration at a location contiguous to an inner wall of the cylinder than at that remote from the wall, i.e. in a less uniform manner, and much time is consumptive in preheating the material on the whole.
To cope with those problems it has been proposed that preheating be effected at elevated temperature. However, this will render the plastic material susceptible to overheat on or nearby the inner cylinder wall, eventually leading to inadequate discoloration and even to serious decomposition.
The foregoing situation of the prior art has lent an impetus for the provision of injection assemblies that can work with great reliability in injection-molding plastics and rubbers.